Aphids:

Aphids are very common pests that attack a variety of plants including orchids and can be found clustering on young shoots or leaves, rarely on flowers or roots.

Aphids seldom kill a plant, but when abundant can reduce the vigor of the plant and can result in stunted plant growth. Leaves often become yellowed due to aphid feeding. Allegedly, some aphid species can transmit viruses when feeding. Like mealy bugs, aphids take in more plant sap than they can absorb and use, and excess is excreted as a clear, sweet, sticky substance known as "honeydew." Honeydew attracts ants, etc. etc.

Aphids are small in size, soft-bodied with long legs and antennae, and their color can vary from yellow to green, gray, red or black. They may have wings, but wingless forms are more common. Aphids have a very short life cycle and can reproduce very quickly. Females can reproduce without mating and can give birth to living young. Aphids will produce wingless generations in the spring, followed by a generation of winged forms. These fly away to other plants.

Many naturally occurring predators prey upon aphids. The most important are: ladybird beetle (adults and larvae), green lacewing larvae, syrphid fly larvae and certain parasitic wasps. The wasps are very effective natural enemies of aphids. They deposit an egg inside the aphid's body, the egg hatches into a larva that consumes the aphid's internal parts. Finally, the wasp larva matures, pupates inside the aphid's dead body, and emerges as an adult wasp through a small exit hole. Aphids, mites and mealy bugs share the same natural predators.

Natural predators and parasitic wasps can provide effective control of aphids. Aphids can be removed by gently rubbing the infested plant part with a cloth or cotton wool, dipped in methylated spirit.

If you must use a pesticide or insecticide, check the label to make sure that "aphids" are listed before any application.